It is a problem with existing wideband radios in basestations that they include ADCs (analog-to-digital converters) which do not have sufficient dynamic range to simultaneously handle properly the large signal strengths from mobiles which are close to the basestation and the small signal strengths from mobiles which are far from the basestation close to the basestation's cell boundary. Typically, when the signal strength becomes too large, the ADC saturates, and outputs a digital signal which contains no useful information.
Some existing systems attempt to overcome this problem through the use of AGC (automatic gain control) in which a variable attenuator is used to insert a variable loss in the analog signal before it is digitized. Due to the dynamic range required and the lower frequency limit of PIN diode switches, AGC cannot be implemented at baseband, and implementing the AGC at RF (radio frequency) would degrade the noise figure of the receiver unduly. A compromise solution involves using the AGC at the IF (intermediate frequency). Such a solution has at least three problems. Firstly, a significant noise figure degradation results due to the limited amount of gain in front of the variable attenuator. Adding more gain in front of the attenuator is not feasible due to the linearity constraint of subsequent components. Secondly, the overload is easiest detected at the digital side of the ADC. By the time the overload is detected, a finite amount of signal is lost due to ADC overload, and clicks are detected at audio. In addition, data is lost in CDPD (cellular digital packet data) and modem applications. Thirdly, the circuit implementation is complex and costly.
In U.S. Pat. No. 5,422,643 which issued Jun. 6, 1995 to Chu et al., an ADC with expanded dynamic range is proposed wherein the analog signal is duplicated in a plurality of channels each with a different attenuation or gain inserted. Then, depending upon the amplitude of the signal, a particular channel is chosen such that the resulting compensated signal will not saturate the ADC but will be close to full scale of the ADC. At the output of the ADC, the digitized signal must be adjusted-to reflect the attenuation or gain inserted. The circuitry for selecting the particular analog channel is complex and requires complicated calibration procedures. Furthermore, there is no suggestion in Chu et al. that this type of ADC would be appropriate for use in a wideband receiver context.